1. Field of the invention
The present invention relates to improvements in devices for use as grinding cups for grinding the hard metal inserts or working tips of drill bits and more specifically, but not exclusively, for grinding the tungsten carbide cutting teeth or buttons of a drill bit and the means for detachably connecting the grinding cups to the grinding machine.
2. Description of the Prior Art
In down-the-hole drilling operations the cutting teeth (buttons) on the drill bits become flattened (worn) after continued use. Regular maintenance of the drill bit by regrinding (sharpening) the buttons to return them to substantially their original profile enhances, the bit life, speeds up drilling and reduces drilling costs. Regrinding should be undertaken when the wear of the buttons is optimally a third to a maximum of one-half the button diameter.
Different manual and semi-automatic grinding machines are known for grinding button bits (see for example U.S. Pat. No. 5,193,312; 5,070,654). In a conventional type of machine a grinding cup having the desired profile is rotated at high speed to grind the carbide button and the face of the bit surrounding the base of the button to return the button to substantially its original profile for effective drilling.
The grinding cups conventionally consist of a cylindrical body having top and bottom surfaces. The bottom or working surface consists of a diamond/metal matrix having a centrally disposed convex recess having the desired profile for the button to be ground. A bevelled rim around the recess removes steel from the face of the bit around the base of the button. An upright hollow stem projects from the top surface.
Water and/or air is provided through the hollow stem and a centrally disposed passageway in the cylindrical body of the grinding cup. A slot or flushing channel in the grinding surface helps disperse the water/air over the button surface being ground. The water/air flushes the surface of the button during grinding. There is a tendency for a nipple to form on the button in the area of the water/air outlet as there is no grinding surface in contact with the button at this point.
The grinding cups are conventionally manufactured by first machining a blank with the upright hollow stem projecting from its top surface. The blank is then pressed into a mould containing a hot diamond/metal mixture. The bottom surface of the blank is heated and bonds to the diamond/metal matrix. Alternatively the diamond/metal matrix can be formed into the grinding section and then bonded either by a shrink fit or with adhesives to a blank.
The grinding cups until recently were conventionally held in the grinding machine by inserting the stem in a chuck for detachable mounting of tools. Special tools such as chuck wrenches, nuts and collets were necessary to insert, hold and to remove the grinding cup into and out of the chuck.
To eliminate the need for chuck wrenches etc. the use of a shoulder drive on the grinding cups was recently developed. A diametrically extending recess at the free end of a hollow drive shaft of the grinding machine co-operates with a shoulder or cam means on the adjacent top surface of the grinding cup. The stem of the grinding cup is inserted into the hollow drive shaft and maybe held in place by one or more O-rings either located in a groove in the interior wall of the drive shaft or on the stem of the grinding cup. See for example Swedish Patent No. B 460,584 and co-pending U.S. patent application Ser. No. 08/305,176 both incorporated herein by reference.
Regardless of whether the grinding cup is retained in a chuck or a shoulder drive is utilized, the grinding cup is rigidly connected to the grinding machine and vibrations generated during the grinding operations are directly transferred to the grinding machine. Excessive vibration can result in:
a) High noise levels during grinding; PA1 b) tiring impacts on the operator's hands and arms where manual grinding apparatus is being used; PA1 c) inconsistent grinding quality, less accuracy of button profile and grinding traces. PA1 d) accelerated wear on chuck and grinding machine components.
In co-pending U.S. patent application Ser. No. 08/305,176 a dampening element was inserted in the channel of the hollow drive shaft adjacent the stem of the grinding cup to help reduce vibrations.